It is well known that ultraviolet light having a wavelength between about 280 nm or 290 nm and 320 nm (UV-B) is harmful to human skin, causing burns that are detrimental to the development of a good sun tan. UV-A radiation, while producing tanning of the skin, also can cause damage, particularly to very lightly colored, sensitive skin, leading to reduction of skin elasticity and wrinkles.
Therefore, a sunscreen composition should include both UV-A and UV-B filters to prevent most of the sunlight within the full range of about 280 nm to about 400 nm from damaging human skin.
The UV-B filters that are most widely used commercially in sunscreen compositions are paramethoxycinnamic acid esters, such as 2-ethylhexyl paramethoxycinnamate, commonly referred to as octyl methoxycinnamate or PARSOL.RTM. MCX, having an ethyl radical extending from the 2 position of the hexyl long chain backbone; oxybenzone; and octyl salicylate.
The UV-A filters most commonly used in commercial sunscreen compositions are the dibenzoylmethane derivatives, particularly 4-(1,1-dimethylethyl)-4'-methoxydibenzoylmethane (PARSOL.RTM. 1789), and 4-isopropyl dibenzoylmethane (EUSOLEX 8020). Other dibenzoylmethane derivatives described as UV-A filters are disclosed in U.S. Pat. Nos. 4,489,057; 4,387,089 and 4,562,067 and 5,670,140, hereby incorporated by reference. It is also well known that the above described and most commonly used UV-A filters, particularly the dibenzoylmethane derivatives, such as PARSOL.RTM. 1789, suffer in photochemical stability when used alone or in combination with the above-described most commercially used UV-B filters. Accordingly, when used alone or when combined with a UV-B filter, such as 2-ethylhexyl paramethoxycinnamate (PARSOL.RTM. MCX), oxybenzone and/or octyl salicylate, the PARSOL.RTM. 1789 becomes less photochemically stable necessitating repeated, frequent coatings over the skin for sufficient UV radiation protection.
In accordance with the principles of the present invention, it has been found, quite surprisingly, that by including a diester and/or polyester of one or more naphthalene dicarboxylic acids of formula (I), into a cosmetic sunscreen formulation containing a UV-A dibenzyolmethane derivative, particularly PARSOL.RTM. 1789, and/or 4-isopropyl dibenzoylmethane (EUSOLEX 8020), the dibenzyolmethane derivative is photochemically stabilized so that the dibenzyolmethane derivative-containing sunscreen composition with or without additional sunscreen agents, such as oxybenzone and/or octyl methoxycinnamate (ESCALOL 567), is more effective for filtering out UV-A radiation; the composition filters more UV-A radiation for longer periods of time; and, therefore, the sunscreen formulation need not be applied to the skin as frequently while maintaining effective skin protection against UV-A radiation.
In accordance with another important advantage of the present invention, it has been found that the diesters and polyesters of naphthalene dicarboxylic acids can also absorb UV light in the most damaging range of about 280-300 nm, especially over the 280 and 295 nm wavelength absorbance peaks shown in FIG. 9, to further boost the SPF of the sunscreen compositions.
By the addition of UV-B filter compounds, such as octyl methoxycinnamate, octyl salicylate, and/or oxybenzone, the cosmetic sunscreen formulation can maintain surprisingly effective skin protection against UV radiation both in the UV-A and UV-B range, with or without common sunscreen additives, such as octocrylene, and/or titanium dioxide. The composition reaches a surprisingly high SPF without solid additives, such as titanium dioxide, thereby providing an exceptionally elegant feel that can be applied easily in a continuous coating for complete coverage and sunscreen protection. The ratio of UV-A to UV-B filter compounds is in the range of about 0.1:1 to about 3:1, preferably about 0.1:1 to about 0.5:1, most preferably about 0.3:1 to about 0.5:1. Quite surprisingly, the preferred compositions of the present invention achieve unexpectedly high SPF, e.g., higher than SPF 12 in one preferred composition, and higher than SPF 20 in another preferred composition, with the addition of surprisingly low amounts of other UV-B and UV-A filters to the PARSOL 1789, and without solid blocking compounds, such as TiO.sub.2.